A solvent-soluble curable fluororesin coating material containing a copolymer of a fluoroolefin having a crosslinkable functional group with a vinyl ether or a vinyl ester (JP-A-59-102962, JP-A-61-57609) presents a coating film excellent in corrosion resistance, durability, stain-removal properties, etc., and has been widely used in recent years in e.g. a heavy duty corrosion prevention, building or industrial field.
However, a solvent coating material releases an organic solvent to the environment during its use and thus is regarded as a factor of environmental pollution. In order to cope with an environmental pollution problem which has been increasingly serious year after year, attention has been drawn to a fluororesin powder coating material
Usually, a powder coating material is produced by melt-kneading a composition having a preliminarily pulverized resin component and other additives blended, followed by pulverization e.g. by a hammer mill, or produced by mixing such a resin composition as dissolved in a solvent, with other additives, followed by spray drying.
A powder coating material containing a polyvinylidene fluoride resin as the main component has a high crystallinity and thus is required to be baked at a high temperature of at least 200° C., and it further has a problem that a coating film having adequate gloss can hardly be obtainable, whereby the application is rather limited.
JP-A-2-60968 proposes a powder coating material containing a copolymer of a fluoroolefin having a crosslinkable functional group with a vinyl ether or a vinyl ester.
Here, there is no disclosure of the glass transition temperature (Tg) of this copolymer, but with a copolymer having a Tg of at most 40° C., the lower the Tg, the lower the blocking resistance. Here, the blocking resistance means a nature to prevent fusion of particles to one another during the storage of the powder coating material. Further, it is required to carry out pulverization at a low temperature at the time of preparation of the powder coating material, whereby a low temperature energy is required only for the purpose of pulverization, thus leading to an increase of costs.
On the other hand, a powder coating material containing as the main material a non-vinylidene fluororesin having a Tg higher than 40° C., is excellent in efficiency in the pulverization and in blocking resistance of the obtained powder, but it has had a problem that when impact resistance higher than a conventional level is required, such a requirement can not be satisfied.